1. Field of the Invention
This invention relates to the formation of printed circuits arrangements involving through holes in a layer of insulation or substrate for enabling electrical inter-connection between conductive material provided on opposite sides of the insulation layer or the substrate.
Present day printed circuit arrangements frequently include a substrate and several layers of electrically conductive planes and/or conductive systems separated and electrically insulated from each other by layers of insulation, and which have mounted thereon circuit elements which operatively connect with the conductor planes.
Hitherto the formation of such circuits has involved the successive application of layers of conductive materials and the insulation materials. In practice, this has been found to lead to relatively low production yields since any production errors in the latter stages of production results in rejection of the complete circuit arrangement.
With a view to overcoming these problems, it has been proposed to divide the construction of the circuit arrangements into at least two principal units.
The first unit comprises a metallic substrate, which is intended to provide for dissipation of heat generated during operation, and which also provides the support for power supply layers. The second unit comprises a flexible plastic film which carries on opposite faces thereof conductive layers which provide continuous layers or conductive patterns which provide the signal connection paths of the circuit arrangement and some of the power connections linking the circuit components connected with the completed circuit or arrangement, and mounted on the second unit.
The substrate of the second component can comprise a layer of polyimide film which, for example, has a thickness of 25 um or 50 um.
The present invention is particularly concerned with the formation of this second unit and more particularly concerned with overcoming certain difficulties which have been found to arise in connection with the formation of through-holes in the then flexible substrate and the subsequent covering of the walls of the holes with an electrically conductive metal layer to provide an electrically conductive path from one end of the hole to the other.
The formation of such plated holes providing electrical continuity from one surface of the substrate to the other has met with considerable difficulty which has been found to stem from the use of a very thin layer of a flexible material.
In a previously used method of preparing the double sided printed circuit arrangements having through holes mechanically to drill, the substrate is covered on both surfaces with a metallized coating, holes are then etched through the metalized coating the the substrate, and the walls of the holes are then plated after the removal of the metalised layers since it had been found that their retention prevented the reliable plating of the walls of the holes. After the hole plating the substrate is reprocessed by application of various layers of metallic material and insulation to provide the required electrical conductor patterns.
In practice, it has been found that the execution of the above process - although enabling the plating of the holes to obtain a satisfactory conductive path introduced a further difficulty in that the removal of the metalised layers allowed the flexible substrate to relax. This relaxation causes unpredictable relative movements of the holes with respect to subsequent masks and the associated conductive layers. Consequently, when the conductor patterns were produced by the conventional photo-resist and mask technique which required the holes to be located in very precise locations in the substrate, the holes were not always correctly positioned with respect to the conductors.
It is an object of the present invention to overcome this difficulty.